1) Field of the Invention
The present invention relates to a waveguide substrate and a high-frequency circuit module suitable for use to convert signals from a circuit substrate to a waveguide in a microwave and millimeter wave band.
2) Description of the Related Art
When a high-frequency signal, particularly, a signal in a short wavelength band such as a millimeter wave, is irradiated from or received by an antenna with a transmitting/receiving circuits, the mode of the signal is converted into a signal propagation mode in a cavity waveguide and the signal is connected, between the transmitting/receiving circuit and the antenna.
Namely, it is known that the transmission loss of a high-frequency signal can be decreased by converting the signal into the signal propagation mode in a cavity waveguide and feeding the signal to the antenna, rather than by directly feeding the signal from the circuit chip such as a transmission circuit to the antenna. As a member for interfacing the signal between a circuit chip or the like configuring the transmitting/receiving circuit and the cavity waveguide, known is one disclosed in Patent Document 1 below.
In Patent Document 1, described is a member 100 shown in a plan view of FIG. 19, for example. In the member 100 shown in FIG. 19, conductive films 102 made of a metal or the like are formed on the both surfaces of a dielectric plate not shown, and a plurality of through conductors 103 which penetrate the dielectric plate and electrically connect between the conductive films 102 on the both surfaces of the dielectric plate are arranged in two columns.
A path enclosed by the through conductors 103 in two columns and the conductive films 102 on the both surfaces of the dielectric plate is configured as a post wall waveguide 104, whereby the signal from the circuit chip (not shown) is fed to a cavity waveguide 105 through the post wall waveguide 104. In concrete terms, the post wall waveguide 104 is closed at one end by a row of through conductors 103c, and a coupling window 104w is formed at a position away a predetermined distance L1 from the row of through conductors 103c. 
The coupling window 104w is formed by removing a part of the conductive film 104 formed on the H plane of the post wall waveguide 104. Incidentally, the cavity waveguide 105 is fixed at a position so that the opening of the cavity waveguide 105 touches the coupling window 104w. Whereby, the high-frequency signal propagated in the post wall waveguide 104 is led to the cavity waveguide 105 through the coupling window 104w. 
At this time, the distance L1 between the coupling window 104w and the row of through conductors 103c is preferably a space for matching in the neighborhood of a quarter wave of the used frequency. Namely, the high-frequency signal propagated in the post wall waveguide 104 can be resonated by the row of through conductors 103c being as the short wall. At this time, by forming the coupling window 104w a distance in the neighborhood of a quarter of the used wavelength away, a component corresponding to the node of a standing wave generated by the resonance can be led from the window 104w to the cavity waveguide 105.
As other techniques relating to this invention, there are techniques described in Patent Documents 2 and 3 below:
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 2003-289201
[Patent Document 2] Unexamined Japanese Patent Application Publication No. 2000-151225
[Patent Document 3] Unexamined Japanese Patent Application Publication No. 2004-15404
However, the member 100 shown in FIG. 19 has the following problem.
The row of through conductors 103c forming the above short wall and the coupling window 104w are formed in separate manufacturing processes, not formed at the same time in the same process. Namely, the row of through conductors 10c is formed in a process in which holes are made by a laser, drill or the like, then a conductive material is filled therein. On the other hand, the coupling window 104w is formed by removing a portion corresponding to the coupling window 104w using a technique such as etching or the like during a process in which the conductive film 102 is formed.
For the above reason, it is difficult to accurately keep a quarter of the used wavelength as the distance L1 that should be secured between the row of through conductors 103c and the coupling window 104w due to manufacturing variation arising in each processes. FIG. 20 is a diagram showing the signal conversion characteristic of a post wall waveguide 104 into a cavity waveguide when the distance L1 between the coupling window 104w and the row of through conductors 103c is changed to three values (La, Lb, Lc; La<Lb<Lc) and the member 100 is manufactured, wherein the horizontal axis represents frequency components of high-frequency signals, whereas the vertical axis represents reflection quantities of the corresponding high-frequency signals. When the reflection quantity shown along the vertical axis is increased, the quantity of the high-frequency signal component led to the post wall waveguide 104 is decreased.
As shown in FIG. 20, when the distance between the coupling window 104w and the row of through conductors 103c is changed, a relatively large variation in the reflection characteristic to the frequency of the high-frequency signal generates. In this case, the propagation characteristic to the cavity waveguide largely changes according to the frequency of a used high-frequency signal. Namely, it is preferable to decrease the frequency dependency to the reflection characteristic in order to improve the characteristics of the post wall waveguide 104.
In Patent Document 1, a two-section waveguide band-pass filter is formed in order to obtain a wide band or a desired passing band width. However, the technique disclosed in Patent Document 1 does not provide a technique for improving the signal conversion characteristic of the post wall waveguide 104 to the cavity waveguide due to an error occurring in the manufacturing of a portion leading the signal to the cavity waveguide, that is, the distance L1 between the row of through conductors 103c and the coupling window 104.
Like the above case, techniques described in Patent Documents 2 and 3 do not provide the technique for improving the signal conversion characteristic of the post wall waveguide 104 to the cavity waveguide due to an error occurring in the manufacturing of a portion leading the signal to the cavity waveguide.
In light of the above problem, an object of the present invention is to improve the signal conversion characteristic of the post wall waveguide to the cavity waveguide, which tends to be degraded due to an error occurring in the manufacturing of a portion leading the signal to the cavity waveguide.